The present invention relates to an arrangement and method for heating gases in a gas circulation duct, in connection with continuously operated sintering. In a sintering furnace, hot gas is fed in above the material bed located on the conveyor belt in order to sinter the material, and part of the gas duct is formed as a burning zone, a burner ring, where the gas is heated. The burner ring comprises at least one burner unit directed inwards from the edges of the burner ring.
In continuously operated sintering, there is currently used a conveyor-type sintering device where a material bed is first formed on the conveyor belt. The material bed to be sintered is normally made of spherical pellets with a weak strength, or of ore finess, which is hardened by means of sintering, so that the pellets or the sinter can be further fed for instance to a smelting furnace without dust problems. Generally a sintering device includes separate zones for the drying, preheating and sintering of the material to be sintered, and for cooling the sintered product, and said successive steps are realized by conducting gas through the material bed and also through the conveyor belt. For example when processing ferroalloy pellets, hot gas is conducted in the sintering zone through the material bed and the belt, so that the bed temperature is raised up to the range of 1000-1600xc2x0 C. At a high temperature, the pellets or the sinter react with the hot gas and are simultaneously hardened. The hardened pellets are cooled by conducting cooling gas through the material bed and the belt.
As was already pointed out, in the sintering device the thermal treatment of the material to be sintered is carried out by means of gas, by placing gas conduits around the sintering belt, in the immediate vicinity of said belt. Thus for instance for a cooling process that takes place at the final end of the belt, gas is fed in from underneath the belt, and gas is sucked from above the belt up to gas circulation ducts, where at least part of the gas is heated, and the heated gas is conducted to the first end of the belt, either to the drying, heating or sintering zone thereof.
The gas used in sintering is traditionally heated by means of separate combustion chambers arranged in the gas circulation duct, where along with the fuel, also the required combustion and spreading air is fed to the burner. The combustion chamber comprises one burner, in which case the adjusting of the temperature is difficult to realize. Normally the combustion chambers are placed in the vertical part of the duct, i.e. near to the surface to be sintered, in which case there are easily created local hot spots, which make it difficult to achieve a homogeneous sintering result. The hot spots also tend to cause damages to the equipment, for instance for the steel belt, the grate and the fireproof linings.
The present invention relates to a method and arrangement for heating gases in a gas circulation duct in connection with continuously operated sintering. In a sintering furnace, hot gas is fed in from above the belt in order to sinter the material located on the belt, and part of the gas duct is formed as a combustion zone, a burner ring, where the gas is heated. At least in part of the gas circulation ducts, there is formed a burner ring, and a separate combustion chamber is not needed. A burner ring comprises at least one burner unit which is directed inwards from the edges of the burner ring, but generally the number of burner rings is at least two. Advantageously the burner units are located on the same circumference, and each unit includes feed ducts for one or several fuels. Each burner ring is provided with at least one ignition burner. When the fuel used for heating has been ignited by means of the ignition burner, there is no need for separately feeding in combustion air, but the combustion air is obtained from the gas contained in the gas circulation duct. By employing the burner ring, there is achieved an even temperature, and local temperature peaks are avoided. The essential novel features of the invention are apparent from the appended claims.
The burner ring according to the invention is advantageously constructed in the horizontal part of the gas circulation duct and sufficiently far from the bed to be sintered. When several burner units are used, there is obtained a large adjustment area that covers the temperature requirements during both the running-up period and normal operation. Generally the burner unit comprises, in addition to the fuel ducts of at least two different fuels, a flame guard, an ignition burner and an inspection opening. Combustion air must be fed separately only in the ignition burner, and the rest of the combustion air is obtained from the gas flowing in the gas duct. However, when several units are placed on one and the same circumference, it is not necessary to provide every unit with for instance a flame guard, ignition burner and inspection opening, but or example one ignition burner can take care of several units.
The interior of the gas circulation duct is made of a fireproof material. At the burner ring, there is advantageously made a small inwardly protuberance, i.e. a constriction. By means of the constriction, the gas flow in the duct is made turbulent, and simultaneously the fuel is mixed in the gas flow. Thus the constriction also evens out the temperature on different sides of the duct, and an even temperature is obtained above the bed to be sintered.
Ignition burners are provided for example with hydraulic control and automatics, so that they are in the gas duct with the flame on only until a given temperature is reached, whereafter the flame is extinguished and they are drawn in, to the protection of the duct masonry and other structures. This means a remarkable increase in the working age of the ignition burners. In the control function, also electricity or pressurized air can naturally be used.